1. Field of the Invention
This invention relates to a birefringence insensitive fiber optic optical coherence domain reflectometry (OCDR) system. In particular, the system is designed to provide a disposable section of non-polarization maintaining optical fiber in the sample arm while achieving high resolution by matching the dispersion between the sample arm and the reference arm.
2. Description of Related Art
Optical coherence domain reflectometry (OCDR) is a technique developed by Youngquist et al. in 1987 (Youngquist et al., xe2x80x9cOptical Coherence-Domain Reflectometry: A New Optical Evaluation Techniquexe2x80x9d, 1987, Optics Letters 12(3):158-160). A similar technique, optical coherence tomography (OCT), was developed and used for imaging with catheters by Swanson et al. in 1994 (See U.S. Pat. Nos. 5,321,501 and 5,459,570). OCDR and OCT have been applied to imaging and diagnoses of biological tissues, such as dental tissue (See U.S. Pat. No. 5,570,182 to Nathel et al.). OCT systems have been miniaturized to enable their use with guidewires. OCDR and guidewire systems are disclosed in WO 99/02113 (PCT/US98/14499) to Winston et al. and U.S. patent application Ser. No. 09/050,571 to Everett et al.
A diagram of a prior art OCDR scanning system is shown in FIG. 1. Light from a low coherence source 10 is input into a 2xc3x972 fiber optic coupler 12, where the light is split and directed into a sample arm 14 and a reference arm 16. An optical fiber 18 in the sample arm 14 extends into a device 20 that scans an object 22. The reference arm 16 provides a variable optical delay. Light input into the reference arm 16 is reflected bade by a reference mirror 24. A piezoelectric modulator 26 may be included in the reference arm 16 with a fixed reference mirror 24, or the modulator 26 may be eliminated by scanning the mirror 24 in the Z-direction. The reflected reference beam from reference arm 16 and the scattered sample beam from sample arm 14 pass back through the coupler 12 to detector 28 (including processing electronics), which processes the signals by techniques that are known in the art to produce a backscatter profile or image on a display unit 30.
Standard fiber optic OCDR systems currently use non-polarization maintaining (non-PM.) fiber throughout, leading to loss of signal and to artifacts associated with mismatches between the polarization states of the light from the reference and sample arms (polarization fading). These mismatches are caused by birefringence in the sample and reference arms and the sample itself.
Several attempts have been made to eliminate this polarization fading through the use of polarization diversity receivers, where the light returning from the sample and reference arms is split into two orthogonal polarization modes each mode is detected by a separate detector. To minimize costs, such system would ideally have non-PM fiber in the sample arm. However all polarization insensitive systems developed to date with non-PM fiber in the sample arm have either suffered from dispersion caused by PM fiber in the reference arm, or variations in the polarization state of light returning from the reference arm, caused by changes in the birefringence of non-PM reference arm fiber.
Co-pending U.S. patent application Ser. No. 09/050,571 to Everett et al. describes a sensing system, shown in FIG. 2, in which the polarization of the light through the system is controlled by polarization maintaining (PM) fibers and optics. Linearly polarized light is introduced into the system either through use of a linearly polarized broadband light source 40 or by placing linear polarizer 42 directly after an unpolarized source 40. The linear polarization of the light is then maintained through the use of PM fibers and a PM fiber optic coupler 44, where the linear polarization is one of the two modes of the PM fiber and PM coupler 44. The polarization state of the light in the reference arm 46 is modified by either a waveplate or a faraday rotator 48 so as to be equally split between the two modes (orthogonal polarizations) of the PM fiber upon reflection. A polarization beam splitter 50 in the detector arm 52 splits the two polarization modes and directs them to two separate detectors 54,56 connected to the image processing and display unit 58.
In one embodiment shown in FIG. 2, the multiplexed optical fibers 60 in the sample arm 62 are polarization maintaining (PM). The sample arm 62 contains a multiplexer 66 for switching between the plurality of fibers 60, allowing sequential spatially distinct regions to be observed consecutively using the OCDR system. The fibers 60 can be oriented such that the light leaving the fibers is linearly polarized at an angle approximately 45xc2x0 relative to the fast axis of birefringence of the sample 64. Alternatively, a quarter waveplate can be placed at the distal end of each fiber 60 to cause the light entering the sample 64 to be circularly polarized. In either case, the total light in all polarization states returning from the sample 64 is determined by summing the signal from the two detectors 54,56. In addition, processing and display unit 58 includes means for ratioing the output signals from detectors 54,56; the birefringence of the sample 64 is determined based on the arc tangent of the ratio of the signals from the two detectors 54,56.
In another embodiment described in U.S. patent application Ser. No. 09/050,571 to Everett et al., the optical fibers 60 in the sample arm 62 are not polarization maintaining (non-PM). In this case, the polarization beam splitter 50 ensures that the polarization state of the light from the reference arm 46 and the sample arm 62 is matched on each detector 54,56, thus eliminating the losses due to depolarization of the light. The light returning from the sample arm 62 is then measured by summing the signals from the two detectors 54,56.
It was found that the hybrid system described above containing non-PM fiber in the sample arm and PM fiber in the reference arm suffered from path length offsets between the two polarization modes, and reduced resolution caused by a difference in chromatic dispersion between the sample arm 62 non-PM fiber and the reference arm 46 PM fiber. Chromatic dispersion causes pulse broadening due to unequal speeds of different wavelength components of light in the reference arm fiber that are not matched by the sample arm non-PM fiber. The difference in the group velocity between the two polarization modes in the reference arm also lead to a path mismatch between the two polarization modes, which causes additional problems.
An alternate design for a fiber optic polarization insensitive OCDR system with non-PM fiber in the sample arm has previously been described (Kobayashi et al, xe2x80x9cPolarization-Independent Interferometric Optical-Time-Domain Reflectometerxe2x80x9d, 1991, J. Lightwave Tech. 9(5):623-628). The reference arm in this system consists of all PM optical fiber, leading to loss of resolution due to mismatched dispersion between the sample and reference arms. The system also requires a specialized 50/50 coupler.
Another design of a polarization insensitive OCDR system is described by Sorin et al. in U.S. Pat. No. 5,202,745. In this design, a linear polarizer in the reference arm is adjusted to compensate for birefringence in the reference arm so as to equal signal powers on each detector in the detector arm in the absence of a signal from the test, or sample, arm. The problem with this approach is that the polarizer needs to be adjusted as the birefringence in the reference arm changes. As the birefringence in the non-PM reference arm fiber is strongly affected by temperature and stress, the system must be recalibrated with each use, and suffers from polarization drift during use.
Despite the problems with the systems described above, there is strong motivation to incorporate non-PM fiber into the sample arm, particularly to accommodate a disposable section at the end of the sample arm that interacts with the sample. For medical applications, the portion of the fiber optic interacting with the patient must be changed for hygienic reasons. The cost of PM fiber and PM fiber connectors makes disposable PM fiber based sensing arms impractical. Thus, a need exists to incorporate non-PM fiber into the sample arm while eliminating the dispersion effects that degrade image resolution.
The present invention addresses the above-mentioned problems and significantly improves on the system described in U.S. patent application Ser. No. 09/050,571 by providing a design for a less expensive, more robust, birefringence insensitive OCDR system that accommodates a disposable non-PM fiber in the sample arm, yet eliminates dispersion issues.
The object of the present invention is to provide a birefringence insensitive fiber optic optical coherence domain reflectometry (OCDR) system containing non-polarization maintaining (non-PM) fiber in the sample arm. Birefringence insensitive systems eliminate signal degradation caused by birefringence. Another object of the present invention is to minimize mismatches in dispersion between the sample and reference arms, while maintaining a disposable section of non-PM fiber in the sample arm. This is accomplished through the use of matching non-PM fiber in the reference arm. A further object of the invention is to provide a portable, robust OCDR system that can be used in medical applications or for non-medical in situ probes. A further object of the invention is to provide a means of incorporating a single mode fiber optical path modulator in the system for providing optical path scanning. A further object of the invention is to provide a means for more efficiently coupling of optical power to and from the sample arm.
In the present invention, the disposable portion of non-PM optical fiber in the sample arm is useful for incorporation into various clinical devices such as catheters, guidewires, and hand-held instruments or probes. The use of non-PM fiber significantly reduces the cost of these devices. Disposable sections of non-PM fiber can be incorporated into both the sample arm and the reference arm to permit convenient replacement of fibers used on patients or to rapidly configure OCDR systems with different path lengths.
Many OCDR systems, particularly in medical applications, require a portion of the sample arm that is either disposable or multiplexed. The use of polarization maintaining (PM) fiber throughout the OCDR system in conjunction with a polarization diversity receiver is beneficial from the standpoint of eliminating signal fading associated with birefringence. The use of non-PM fiber in the disposable or multiplexed portion of the sample arm is preferable due to its significantly reduced cost. However, the use of non-PM fiber in the sample arm with PM fiber in the reference arm causes the OCDR system to suffer loss of resolution due to mismatches in dispersion between the two arms.
These problems are overcome in the present invention by matching the dispersion in the sample arm (having a section of non-PM fiber) with dispersion in the reference arm in the OCDR system. It is an object of this invention to accomplish this dispersion matching using a section of non-PM fiber in the reference arm. Birefringence effects in this non-PM fiber are then eliminated using a faraday rotator between the non-PM fiber and reference miror. This faraday rotator rotates the polarization of the light so that light is returned through the fiber at 90xc2x0 to its polarization state just prior to the faraday rotator, thus cancelling the effects of birefringence in the fiber. In one embodiment of this invention an additional faraday rotator, which rotates the polarization of light by approximately 45xc2x0 upon double passing is placed between the PM and non-PM fiber in the reference arm. This faraday rotator causes the returning light from the reference arm, which was initially in a single polarization mode of the PM fiber, to be split between the two polarization modes. The polarization diversity receiver then consists of two or more detectors, which detect light in each of the two polarization modes of the fiber. In an alternative embodiment of the invention, essentially all or all fiber used in the reference arm and the sample arm is non-PM fiber. Once again, a polarization diversity receiver collects the light in each of two orthogonal polarization modes. This design is significantly less expensive, but suffers from polarization drift in the source and detector arm optical fibers, which are not double passed. This can be minimized by using optical fibers that are as short as possible.
In yet another embodiment, the detector arm and source arm of the system can be combined to form a more efficient system. In this case, use of coupler which couples more than 50% of the light from the source/detector arm to the sample arm and back can increase the sensitivity of the system. Use of a 90/10 coupler in this system would allow up to 81% of the light from the source to interact with the sample and return versus a maximum of 25% obtained with a 50/50 coupler if the source and detector are in separate arms. The combination source/detector arm contains a plurality of beamsplitters and detectors to collect the light.
The present invention is useful for medical applications, particularly in ophthalmology, dentistry, and cardiology, as it eliminates birefringence effects in both the tissue sample and fiber optics. Birefringence in biological tissues, such as the eye or dental tissue, leads to artifacts in images with conventional OCT systems. Artifacts and signal fading associated with birefringence in optical fibers is also a serious problem in clinical systems, particularly in catheter or guidewire based OCDR imaging systems. The catheter or guidewire in these systems must be replaced for each patient. The present invention provides the ability to incorporate non-PM optical fiber in those systems to significantly lower costs and facilitate the replacement of portions of the sample arm, while eliminating artifacts and signal losses due to polarization fading.
The present invention can also be used in non-medical applications where the fiber or probe in the sample arm becomes damaged or contaminated by the sample being imaged, and thus the fiber must be replaced repeatedly. The use of non-PM fiber in such systems is therefore advantageous and cost-effective. The invention can be used as a single point probe to examine defects in fiber optics, for example, or the sample arm can be scanned to form two-dimensional images or depth-resolved images. Other objects, features, and advantages of the present invention will become apparent from the following description and accompanying drawings.